Method of transmitting data in multiple antenna system

ABSTRACT

A method of transmitting data in a wireless communication system comprises receiving feedback data on an uplink data channel, the feedback data comprising a precoding matrix indicator (PMI), wherein the value of the PMI corresponds to an index in a codebook, transmitting a precoding scheme for downlink data on a downlink control channel, wherein the preceding scheme is determined as one of at least two of a transmit diversity irrespective of the received PMI, an acknowledgement indicating preceding according to the received PMI and a new PMI indicating that it is used in precoding downlink data to be transmitted, and transmitting the downlink data on a downlink data channel after applying precoding according to the determined preceding scheme.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. Provisionalapplication Ser. No. 60/946,130 filed on Jun. 25, 2007, U.S. Provisionalapplication Ser. No. 60/978,140 filed on Oct. 8, 2007, U.S. Provisionalapplication Ser. No. 61/025,304 filed on Feb. 1, 2008, Korean PatentApplication No. 10-2007-0081913 filed on Aug. 14, 2007, Korean PatentApplication No. 10-2007-0118166 filed on Nov. 19, 2007 and Korean PatentApplication No. 10-2008-0056001 filed on Jun. 13, 2008 which areincorporated by reference in its entirety herein.

BACKGROUND

1. Technical Field

The present invention relates to wireless communication, and morespecifically, to a method of transmitting data in a multiple antennasystem.

2. Related Art

Wireless communication systems are widely used to provide various typesof communications. For example, voices and/or data are provided by thewireless communication systems. General wireless communication systemsprovide multiple users with one or more shared resources. For example,the wireless communication systems may use a variety of multiple accesstechniques such as code division multiple access (CDMA), time divisionmultiple access (TDMA), and frequency division multiple access (FDMA).

Orthogonal frequency division multiplexing (OFDM) uses a plurality oforthogonal subcarriers. OFDM uses the characteristic of orthogonalitybetween Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform(FFT). A transmitter transmits data after performing IFFT on the data. Areceiver restores original data by performing FFT on a received signal.The transmitter uses IFFT to combine multiple subcarriers, and thereceiver uses corresponding FFT to separate the multiple subcarriers.According to OFDM, complexity of the receiver may be lowered in afrequency selective fading environment of wideband channels, andspectral efficiency may be enhanced through selective scheduling or thelike in a frequency domain by utilizing different channelcharacteristics of subcarriers. Orthogonal frequency division multipleaccess (OFDMA) is a multiple access scheme based on OFDM. According toOFDMA, efficiency of radio resources may be enhanced by assigningdifferent subcarriers to multiple users.

Recently, multiple input multiple output (MIMO) systems are spotlightedin order to maximize performance and communication capacity of wirelesscommunication systems. The MIMO technique is a method that can improvetransmission efficiency of transmit and receive data by employingmultiple transmit antennas and multiple receive antennas, getting out ofusing one transmit antenna and one receive antenna used up to thepresent. The MIMO system is also referred to as a multiple antennasystem. The MIMO technique does not depend on a single antenna path inorder to receive one whole message, but applies a technique that gathersfragmented data segments received through a plurality of antennas andcompletes a message. As a result, data rate may be improved within aspecific range, or a system range may be increased for a specific datarate.

Hereinafter, downlink means transmission from a base station to a userequipment, and uplink means transmission from the user equipment to thebase station.

Generally, the base station schedules radio resources of uplink anddownlink in a wireless communication system. User data or controlsignals are carried on the uplink radio resources and downlink radioresources. A channel carrying user data is referred to as a datachannel, and a channel carrying control signals is referred to as acontrol channel. The control signals include various types of controlsignals needed for communications between the base station and the userequipment. For example, control signals needed for scheduling radioresources in a multiple antenna system include channel quality indicator(CQI), rank indicator (RI), precoding matrix indicator (PMI), and thelike. The user equipment transmits uplink control signals such as CQI,RI, PMI and the like to the base station, and the base station schedulesradio resources for uplink and downlink based on the control signalsreceived from a plurality of user equipments. The base station informsthe user equipment of RI, PMI, modulation and coding scheme (MCS) of thescheduled radio resources through the downlink control signals.

Errors may occur in uplink control signals transmitted from the userequipment to the base station in the process of transmission. If anerror occurs in an uplink control signal, it may cause difficulties toschedule radio resources. However, it is not clearly suggested how tomake up for an error when the error occurs in the uplink control signalthat is needed by the base station for scheduling radio resources.

Therefore, there is a need for a method of preparing for errors thatoccur in uplink control signals.

SUMMARY

The present invention provides a method for scheduling and transmittingdata, which can cope with errors that may occur in uplink controlsignals needed for scheduling radio resources.

In an aspect, a method of transmitting data in a wireless communicationsystem comprises receiving feedback data on an uplink data channel, thefeedback data comprising a preceding matrix indicator (PMI), wherein thevalue of the PMI corresponds to an index in a codebook, transmitting apreceding scheme for downlink data on a downlink control channel,wherein the preceding scheme is determined as one of at least two of atransmit diversity irrespective of the received PMI, an acknowledgementindicating preceding according to the received PMI and a new PMIindicating that it is used in preceding downlink data to be transmitted,and transmitting the downlink data on a downlink data channel afterapplying preceding according to the determined preceding scheme.

In another aspect, a method of processing data in a wirelesscommunication system comprises configuring feedback data comprising atleast one PMI, wherein the value of a PMI corresponds to an index in acodebook, reporting the feedback data on an uplink data channel,receiving a preceding scheme for downlink data on a downlink controlchannel, wherein the preceding scheme is determined as a transmitdiversity irrespective of the reported PMI or a preceding matrix whichis used to precode the downlink data, and receiving the downlink data ona downlink data channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows a transmitter having multiple antennas.

FIG. 3 shows a receiver having multiple antennas.

FIG. 4 is an exemplary view showing granularities of control signals forradio resource allocation according to an embodiment of the presentinvention.

FIG. 5 is a flowchart illustrating a method of transmitting dataaccording to an embodiment of the present invention.

FIG. 6 is a flowchart illustrating a method of determining whether toapply PMI according to an embodiment of the present invention.

FIG. 7 is an example of a graph showing throughputs with respect toerrors in feedback data.

FIG. 8 is another example of a graph showing throughputs with respect toerrors in feedback data.

FIG. 9 is still another example of a graph showing throughputs withrespect to errors in feedback data.

FIG. 10 is still another example of a graph showing throughputs withrespect to errors in feedback data.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system.

Wireless communication systems are widely deployed to provide a varietyof communication services such as voices, packet data, and the like.

Referring to FIG. 1, the wireless communication system comprises userequipments (UEs) 10 and a base station (BS) 20. A UE 10 can be fixed ormobile and referred to as another terminology, such as a mobile station(MS), a user terminal (UT), a subscriber station (SS), a wirelessdevice, or the like. Generally, the BS 20 is a fixed stationcommunicating with the UE 10, which can be referred to as anotherterminology, such as a node-B, base transceiver system (BTS), accesspoint, or the like. There are one or more cells within the coverage of aBS 20.

Any multiple access technique may be applied to the wirelesscommunication system. A variety of multiple access techniques such ascode division multiple access (CDMA), time division multiple access(TDMA), frequency division multiple access (FDMA), and orthogonalfrequency division multiple access (OFDMA) may be used. For clearexplanation, a wireless communication system based on OFDMA will bedescribed hereinafter.

A wireless communication system may be a multiple antenna system. Themultiple antenna system may be a multiple input multiple output (MIMO)system. Or, multiple antenna system may be a multiple input singleoutput (MISO) system, a single input single output (SISO) system, or asingle input multiple output (SIMO) system. The MIMO system uses aplurality of transmit antennas and a plurality of receive antennas. TheMISO system uses a plurality of transmit antennas and a single receiveantenna. The SISO system uses a single transmit antenna and a singlereceive antenna. The SIMO system uses a single transmit antenna and aplurality of receive antennas.

FIG. 2 shows a transmitter having multiple antennas.

Referring to FIG. 2, the transmitter 100 comprises a scheduler 110,channel encoders 120-1 to 120-K, mappers 130-1 to 130-K, preprocessors140-1 to 140-K, and a multiplexer 150. The transmitter 100 alsocomprises Nt (Nt>1) transmit antennas 190-1 to 190-Nt. The transmitter100 may be a part of the BS in downlink, and the transmitter 100 may bea part of the UE in uplink.

The scheduler 110 receives data from N users and outputs K streams to betransmitted at a time. The scheduler 110 determines users and data ratesto be transmitted through available radio resources using channelinformation of each user. The scheduler 110 extracts the channelinformation from feedback data and selects a code rate, MCS, and thelike.

The channel information may include channel state information (CSI), achannel quality indicator (CQI), user priority information, and thelike. The CSI includes a channel matrix, a channel correlation matrix, aquantized channel matrix, a quantized channel correlation matrix, andthe like between a transmitter and a receiver. The CQI includes signalto noise ratio (SNR), signal to interference and noise ratio (SINR), andthe like. The user priority information is information on a priority ofa user according to a user level and the like.

Available radio resources allocated by the scheduler are radio resourcesused for transmitting data in a wireless communication system. Forexample, time slots are resources in a TDMA system, codes and time slotsare resources in a CDMA system, and subcarriers and time slots areresources in an OFDMA system. In order to avoid interference with otherusers within the same cell or sector, each resource may orthogonallydefined in a time, code, or frequency domain.

Each of the channel encoders 120-1 to 120-K encodes an input stream in apredetermined coding scheme and forms coded data. Each of the mappers130-1 to 130-K maps the coded data to a symbol representing a locationon a signal constellation. The symbol is referred to as informationsymbol. Any kind of modulation scheme can be used, including m-phaseshift keying (m-PSK) and m-quadrature amplitude modulation (m-QAM). Forexample, the m-PSK may be BPSK, QPSK, or 8-PSK. The m-QAM may be 16-QAM,64-QAM, or 256-QAM.

The preprocessors 140-1 to 140-K perform preceding on inputtedinformation symbols u₁, . . . , u_(k) and generate input symbols x₁, . .. , x_(k). The preceding is a technique for performing preprocessing onthe information symbols to be transmitted, and the precoding techniqueincludes random beamforming (RBF), zero forcing beamforming (ZFBF), andthe like for creating input symbols by applying a weight vector, apreceding matrix, or the like to the information symbols.

The multiplexer 150 assigns the input symbols x₁, . . . , x_(k) toappropriate subcarriers and multiplexes the symbols according to a user.The multiplexed symbols are modulated and transmitted through thetransmit antennas 190-1 to 190-Nt.

FIG. 3 is a block diagram showing a receiver having multiple antennas.

Referring to FIG. 3, a receiver 200 comprises a demodulator 210, achannel estimator 220, a post-processor 230, a demapper 240, a channeldecoder 250, and a controller 260. The receiver 200 also comprises Nr(Nr>1) transmit antennas 290-1 to 290-Nr. The receiver 200 may be a partof the UE in downlink, and the receiver 200 may be a part of the BS inuplink.

Signals received through the receive antennas 290-1 to 290-Nr aredemodulated by the demodulator 210. The channel estimator 220 estimatesa channel, and the post-processor 230 performs post-processingcorresponding to the preprocessors 140-1 to 140-K. The demapper performsdemaping input symbols to coded data, and the channel decoder 250decodes the coded data and restores original data. The controller 260feeds back feedback data including CSI, CQI, user priority information,and the like to the transmitter.

Hereinafter, granularities applied to control signals for radio resourceallocation in a wireless communication system will be described.

FIG. 4 is an exemplary view showing granularities of control signals forradio resource allocation according to an embodiment of the presentinvention.

Referring to FIG. 4, user data and control signals are transmitted in aframe comprising a plurality of resource blocks. The frame may include aplurality of OFDMA symbols in the time domain and a plurality ofresource blocks in the frequency domain. A resource block is a basicunit of radio resource allocation and includes a plurality of contiguoussubcarriers. A subcarrier may be a data subcarrier on which user data orcontrol signals are carried or a pilot subcarrier on which pilot signalsare carried. Pilot signals for each antenna may be carried on the pilotsubcarriers in the multiple antenna system. Data subcarriers and pilotsubcarriers may be arranged in a variety of configurations within theresource block. A transmission time interval (TTI) is a time intervalneeded to transmit a frame.

A frame may be divided into a variety of granularities such as awholeband (WB), PMI band (PB), subband (SB), and the like. The SB is afrequency band on which at least one user data or control signal may beloaded. The SB may include one or more resource blocks. The PB includesone or more adjacent subbands. The PB may have a size that is an integertimes larger than the subband. The WB represents all SBs correspondingto system bandwidth. Comparing the sizes of these bands, it may be thatSB≦PB≦WB.

A frame may be divided into best bands (BB) and residual bands (RB) inthe frequency domain according to a transmission scheme of a controlsignal. The BB indicates at least one subbands selected from thewholeband. The RB indicates remained subbands after excluding the bestbands from wholeband. For example, if it is assumed that CQI istransmitted in a Best-M method (M=2), the CQI is calculated for eachsubband, and two subbands having the largest CQI values among CQIs onrespective subbands are selected. The selected two subbands are bestbands, and the other subbands are residual bands. CQIs on the two bestbands are transmitted as they are. An average of CQIs on all subbandsbelonging to the residual bands is calculated, and the average value maybe transmitted as a CQI on the residual bands. Or, CQIs on the two bestbands are averaged and transmitted as an average CQI on the best bands,and also CQIs on the residual bands may be averaged and transmitted asan average CQI on the residual bands. Or, when a CQI on each of the bestbands or an average CQI is transmitted as the CQI on the best bands, anaverage CQI on the wholeband may be transmitted.

The whole frequency band is divided into a variety of granularities inorder to reduce overhead caused by control signaling and efficientlytransmit the control signals. For example, it is effective to obtain andtransmit a CQI for each subband to provide a service of further superiorQuality of Service (QoS) to a plurality of UEs. However, if CQIs on allsubbands are transmitted, the overhead increase. Therefore, severalsubbands having a high CQI value are selected as best bands, and theCQIs of the best bands are transmitted. Only an average value istransmitted as the CQI on the residual bands.

A precoding matrix indicator (PMI) is control information needed forpreprocessing and post-processing user data. Since the PMI affects QoSof a wireless communication system less than the CQI does, it iseffective to obtain and transmit a PMI on each PMI band having agranularity larger than the subband. The size of the PMI band may beequal to or larger than the subband. A PMI may be obtained on eachsubband, and a PMI on the best bands may be transmitted. In addition,one PMI may be obtained and transmitted on the wholeband. A PMI on aspecific band selected from the wholeband is referred to as a frequencyselective PMI. A PMI on the wholeband is referred to as a frequency flatPMI. The frequency flat PMI may be transmitted on a control channel or adata channel. The frequency selective PMI may be transmitted on a datachannel. An example of the data channel is a physical uplink sharedchannel (PUSCH), and the control channel is a physical control channel(PUCCH). It is since that the frequency selective PMI is variouslydetermined depending on the number of selected specific bands. In somecases, the frequency selective PMI and the frequency flat PMI may betransmitted together, it is called multiple PMIs. Accordingly, it isdifficult to transmit all of the multiple PMIs on a control channel.

A rank indicator (RI) represents an independent channel that can bemultiplexed by multiple antennas, and it is sufficient to obtain andtransmit an RI by the unit of the wholeband WB.

The configuration of the frame and the bands of a variety ofgranularities included in the frame as described above are merely anexample, and the size and number of respective bands may be variouslymodified and applied.

FIG. 5 is a flowchart illustrating a method of transmitting dataaccording to an embodiment of the present invention.

Referring to FIG. 5, in step S110, a BS transmits a request message forrequesting feedback data to a UE. The request message may be transmittedon a downlink control channel which can be called as a physical downlinkcontrol channel (PDCCH). The request message may include uplinkscheduling information which includes an uplink radio resourceassignment to be used to transmit the feedback data and an indicator toindicate transmission of the feedback data.

In step S115, the UE generates the feedback data. The feedback data mayinclude at least one CQI, at least one PMI and one RI. The feedback datamay be generated in various form according to a report type. The reporttype indicates which forms of CQI, PMI and rank is included in thefeedback data. The report type may be given by a radio resource control(RRC) message.

Table 1 shows an example of report types for the feedback data.

TABLE 1 Report Best-M Average type Bitmap RI CQI CQI PMI A comprised WBSB SB PB B comprised WB SB WB PB C comprised WB SB OL PB D comprised WBWB WB PB E comprised WB WB OL PB F comprised WB OL OL —

‘Bitmap’ indicates which subbands are selected among a plurality ofsubbands. That is, selected subbands or PMI bands may be indicated usingthe bitmap. For example, when 6 subbands are expressed by a bitmap of 6bits and the first and third subbands are selected, the bitmap may berepresented as ‘101000’. Or, a plurality of control signals may bedistinguished using the bitmap. For example, a control signal having asequence of one RI, two CQIs of best bands, one average CQI of residualbands, and three PMIs of PMI bands is expressed by a bitmap of 7 bits.If a bitmap is given like ‘0111000’, it means that the control signalcomprises two CQIs of best bands and one CQI of residual bands. Or, whenN best CQIs of M subbands are transmitted or a CQI of the whole band istransmitted in Best-M, a bitmap of null bits may be transmitted.

‘RI’ may be calculated over the whole band WB and corresponds to thenumber of useful transmission layer. A CQI is calculated for eachtransmission layer.

‘Best-M CQI’, i.e., a CQI for M best bands, and ‘Average CQI’ may becomprised in the feedback data as a value for a subband or the wholeband according to each report type. The best-M CQI may be referred to asbest band CQI. One CQI for each subband may be referred to as frequencyselective CQI or subband CQI. One CQI over the whole band may bereferred to as frequency flat CQI or whole band CQI.

In report type ‘A’, ‘Best-M CQI’ is CQIs of M subbands selected bydescending power of CQI values of a plurality of subbands or one CQI ofM subbands. ‘Average CQI’ is an average CQI of residual bands. The CQIfor M subbands may have differential CQI value with respect to theaverage CQI.

In report type ‘B’, ‘Best-M CQI’ is CQIs of M subbands selected bydescending power of CQI values of a plurality of subbands or a CQI of Msubbands. ‘Average CQI’ is an average value CQI of the whole band WB.The CQI for M subbands may have differential CQI value with respect tothe average CQI.

In report type ‘C’, ‘Best-M CQI’ is CQIs of M subbands selected bydescending power of CQI values of a plurality of subbands or a CQI of Msubbands. ‘Average CQI’ is applied to an open loop (OL), which meansthat a CQI for residual bands is not transmitted. The open loop meanstransmitting data without considering feedback data.

In report type ‘D’, ‘Best-M CQI’ and ‘Average CQI’ are respectively anaverage CQI for the whole band. In report type ‘E’, ‘Best-M CQI’ is anaverage CQI for the whole band, and ‘Average CQI’ is not transmitted ortransmitted at further longer periods. In report type ‘F’, ‘Best-M CQI’and ‘Average CQI’ are not transmitted.

‘PMI’ is an index of a preceding matrix selected from a codebook. PMImay be selected over a PMI band (PB) and/or the whole band (WB). The PMIband may have a granularity equal to or larger than the subband.Granularity of a PMI band may be variously determined according to thereport type of the feedback data. A PMI for at least one subbandselected among the wholeband is referred to as a frequency selectivePMI, and a PMI on the wholeband is referred to as a frequency flat PMI.A plurality of subbands may be selected and a plurality of frequencyselective PMIs may be transmitted on an uplink data channel. Thefrequency selective PMI corresponds to an index of a codebook of eachsubband. Multiple PMIs selected from the codebook of each subband isreferred to as a frequency selective PMI. One frequency flat PMI may betransmitted on an uplink control channel.

The types of control signals included in the feedback data are merely anexample and not a limit. For example, ‘PMI’ may be given as a value on asubband or the wholeband, and accordingly, the types of control signalsincluded in the feedback data may be diversely modified. The feedbackdata may be configured in a combination of various types of PMIs andCQIs. For example, the feedback data may be configured with a pluralityof PMIs on each of a plurality of subbands and one wholeband CQI on aplurality of subbands. Or, the feedback data may be configured with onePMI on a plurality of subbands, one CQI on the wholeband and one CQI onbest bands selected among a plurality of subbands. Or, the feedback datamay be configured with one PMI on a plurality of subbands, one PMI onbest bands, one CQI on the whole band and one CQI on the best bands.

In step S120, the UE transmits the feedback data to the BS. The feedbackdata may be transmitted on a physical uplink control channel (PUCCH) orphysical uplink shared channel (PUSCH). The PUCCH or PUSCH may beallocated to the uplink radio resource assignment in the requestmessage. When feedback data includes one or two of a CQI, a PMI and aRI, the feedback data may be transmitted on the PUCCH. On the contrary,when feedback data includes a CQI, a PMI and a RI, the feedback data maybe transmitted on the PUSCH due to limited capacity of the PUCCH. Thefeedback data may be periodically transmitted at transmission intervalsof uplink control signals, and the transmission interval is specified bythe BS or previously promised between the BS and the UE.

When only CQIs or PMIs and CQIs are transmitted through feedback data, afrequency flat CQI and/or a frequency flat PMI may be transmitted usingrelatively a small amount of radio resources. When CQIs and/or PMIs aretransmitted on a control channel that uses a small amount of radioresources, i.e., on a control channel that is restricted in allocatingradio resources, the CQIs and/or PMIs may be transmitted without cyclicredundancy check (CRC).

It may be difficult to transmit frequency selective PMIs on a controlchannel that is restricted in allocating radio resources. When frequencyselective PMIs are transmitted on a control channel, an indicator for aplurality of selected subbands should be assigned. This may be a bigoverhead on the control channel. For example, in the case of a controlchannel having radio resources that can transmit a limited message of 10to 20 bits, there is a limit even in transmitting only CQIs. When CQI,PMI, and RI are transmitted together, it will be a burden inserting anerror detection code such as CRC. Accordingly, it is desirable totransmit a frequency flat PMI on a control channel and frequencyselective PMIs on a data channel.

In step S120, The BS detects an error in the feedback data received fromthe UE. When PMI is transmitted on an error detection channel, the BSmay confirm whether there is an error in the feedback data. The errordetection channel is a channel that can detect whether there is an errorin data by attaching CRC to transmission data. The error detectionchannel is a data channel that can sufficiently use radio resources. Ifthere is an error in the feedback data, the BS determines whether to usethe PMI transmitted from the UE. A type of a confirm message to betransmitted later may be changed depending on whether there is an errorin a bitmap, PMI, and the like of the feedback data.

Table 2 shows an example of allocating radio resources according to areport type of an uplink control signal. It shows that which PMI will beapplied to radio resources that are to be allocated to the UE, assumingthat an error occurs in the bitmap or PMI.

TABLE 2 Allocation type of radio Report Arbitrary resources typeFeedback data Best band Band 1 A — SB SB 2 B No Error SB WB 3 BitmapError WB WB 4 PMI Error OL OL Bitmap and PMI Error 5 C — SB OL 6 D NoError WB WB Bitmap Error 7 PMI Error OL OL Bitmap and PMI 8 E — WB OL 9F No Error OL OL Bitmap Error

‘Arbitrary band’ means bands other than best bands selected from thewholeband. That is, the best bands are bands where radio resources areallocated to users, and the arbitrary band means bands other than thebest bands. When there is no error in feedback data, the BS allocatesradio resources by applying a PMI of the same type as that of the bestbands specified by the UE. When there is an error in the bitmap of thefeedback data, since the BS does not know subbands specified by the UE,the BS allocates radio resources by applying a PMI on the wholeband WBor applying an open loop (OL) MIMO scheme. Hereinafter, the open loopMIMO scheme is referred to as a secondary MIMO transmit scheme. The BSallocates radio resources by applying a primary MIMO transmit schemewhen no error is detected in the feedback data and by applying thesecondary MIMO transmit scheme when there is an error in PMI.

Here, the primary MIMO transmit scheme means a transmit scheme that usesinformation included in the current feedback data, and the secondaryMIMO transmit scheme means a transmit scheme that does not use a PMIincluded in the current feedback data. The primary MIMO transmit schemeis that the BS uses information included in the feedback data. Forexample, the BS scrambles transmission signals in a spatial, time, orfrequency domain using a rank or PMI included in the feedback data andtransmits the scrambled signals through multiple antennas. The secondaryMIMO transmit scheme is that the BS does not use the PMI included in thefeedback data where an error is detected. In secondary MIMO transmitscheme, the BS scrambles transmission signals in a spatial, time, orfrequency domain in a previously specified MIMO scheme and transmits thescrambled signals through multiple antennas. The secondary MIMO transmitscheme may be used temporarily for a predetermined time period. If noerror is detected in the feedback data thereafter, the primary MIMOtransmit scheme may be used.

When an error is detected in the feedback data, the previous feedbackdata may be used. When a current channel state is not abruptly changedfrom the previous channel state, a MIMO transmit scheme may bedetermined using information contained in the previously receivedfeedback data. For example, if an error is detected in the feedback datawhen CQI/PMI is transmitted in report type ‘A’, ‘B’, or ‘C’, the BS mayuse the latest feedback data that does not have an error amongpreviously received feedback data. The transmission interval of the usedlatest feedback data may be informed through an indicator.

At rank 1, space-time coding (STC) such as a space frequency block code(SFBC) and space time block code (STBC), cyclic delay diversity (CDD),frequency switched transmit diversity (FSTD), time switched transmitdiversity (TSTD), or the like may be used as the secondary MIMO transmitscheme. At rank 2 or higher, spatial multiplexing (SM), generalizedcyclic delay diversity (GCDD), selective virtual antenna permutation(S-VAP), or the like may be used as the secondary MIMO transmit scheme.SFBC is a technique that can secure both a diversity gain and a multipleuser scheduling gain in a corresponding dimension by efficientlyapplying selectivity in the spatial and frequency domains. STBC is atechnique that applies selectivity in the spatial and time domains. FSTDis a technique that distinguishes signals transmitted through multipleantennas by frequency, and TSTD is a technique that distinguishessignals transmitted through multiple antennas by time. The spatialmultiplexing is a technique for enhancing a data rate by transmittingdifferent data through each of antennas. GCDD is a technique forapplying selectivity in the time and frequency domains. S-VAP is atechnique that uses a single preceding matrix, which includes multicodeword (MCW) S-VAP for scrambling multi codewords among antennas inthe spatial diversity or spatial multiplexing and single codeword (SCW)S-VAP using a single codeword. The secondary MIMO transmit scheme mayuse only a certain codebook among a plurality of codebooks.

Section 8.4.8 of Institute of Electrical and Electronics Engineers(IEEE) standard 802.16-2004, “Air Interface for Fixed Broadband WirelessAccess Systems”, may be referenced as an example of space-time coding(STC). Section 5.3.4.1 of 3GPP TS 36.211 V1.1.0 (2007-05), “PhysicalChannel and Modulation”, may be referenced as an example of CDD. KoreanPatent Application No. 10-2007-0069770 (Jul. 11, 2007) applied by thepresent inventor may be referenced as an example of GCDD.

In step S130, The BS informs the UE of a MIMO scheme. The MIMO scheme istransmitted through a confirm message. The confirm message indicateswhether the primary MIMO transmit scheme or the secondary MIMO transmitscheme is used. The MIMO scheme indicates whether the PMI included inthe feedback data are used as they are and whether a transmit diversityis used regardless of the PMI included in the feedback data. When thecontrol signals are used as they are transmitted from the UE to the BS,the BS does not need to inform the UE of details of the control signalsagain, but transmits only an acknowledgement message. Particularly,except the case where an error occurs in the feedback data transmittedby the UE or the BS specifies to use another PMI for the reason ofscheduling, the PMI transmitted by the UE is used as is. The UE selectsan optimal PMI based on channel information, channel state information(CSI), and the like between the BS and the UE. Generally, since thechannel state information has a large amount of data, it is nottransmitted to the BS. The CQI transmitted by the UE is calculated andquantized in accordance with the PMI. If the PMI is changed, the CQIshould be recalculated and changed. However, if there is no PMItransmitted from the UE, the BS may not recalculate a PMI. And if anarbitrary PMI is used, QoS may be further degraded.

On the other hand, the confirm message may indicate a new PMI. Theconfirm message may be a message for indicating a PMI that istransmitted lately on a data channel from the UE. Or, the confirmmessage may indicate a previously specified PMI. The confirm message maybe a response message for a frequency selective PMI that is transmittedon an error detection channel.

When the BS determines that there is an error in the feedback data, itcan use a previously specified secondary MIMO transmit scheme. The BSmay select a secondary MIMO transmit scheme instead of using the PMItransmitted by the UE. The BS may transmit a confirm message includingan indicator that indicates the secondary MIMO transmit scheme. Forexample, when an error is detected in feedback data including all kindsof feedback information such as RI, bitmap, PMI, CQI, and the like, theCQI included in the feedback data is unreliable. If downlink data shouldbe transmitted even in this case, a previously specified secondary MIMOtransmit scheme may be used. When control information is transmitted ona control channel in a secondary MIMO transmit scheme, since channelstate information reported to transmit control information can be used,unnecessary retransmissions performed due to a CQI mismatch may bereduced.

When there is no previously specified secondary MIMO transmit schemealthough it is determined that the feedback data has an error, the BSselects a PMI on the wholeband and informs the UE of the PMI. Forexample, an error may occur in the feedback data when a frequencyselective PMI and a frequency selective CQI on best bands aretransmitted on an uplink data channel for transmitting PMIs and CQIs.The BS may specify and use a PMI on the wholeband instead of thefeedback data having an error. The BS may enhance reliability of datatransmission by informing the UE of the PMI on the wholeband through anindicator.

An ACK (Acknowledgment)/NACK (Non-Acknowledgment) signal of one bit thatmerely indicates whether an uplink control signal is applied may be usedas a confirm message. For example, an ACK for a PMI transmitted from theUE means that the PMI is used, and a NACK means that the PMI transmittedfrom the UE is not used. When the BS determines not to use the PMItransmitted from the UE, the BS may further inform the UE of informationon a PMI that will be applied. Or, the confirm message may be expressedwith two bits in order to use a previously transmitted PMI. For example,it may be that the latest transmitted PMI is used if the confirm messageis ‘00’, the PMI transmitted from the UE before the second transmissioninterval is used if the confirm message is ‘01’, and the PMI transmittedfrom the UE before the third transmission interval is used if theconfirm message is ‘10’. If the confirm message is ‘11’, it may be thatthe PMI transmitted from the UE is not used. The meanings of the confirmmessage are merely an example, and those skilled in the art may modifyand apply the meanings in an apparent form.

In step S140, the BS transmits downlink data to the UE on a downlinkdata channel. It is assumed that the BS determines not to use the PMItransmitted by the UE and transmits a confirm message including a NACKsignal. When the PMI transmitted from the UE is not used, a secondaryMIMO transmit scheme is applied to communications between the BS and theUE. For example, if the rank is 1 and the secondary MIMO transmit schemeis SFBC, the UE, received the confirm message of a NACK signal from theBS, receives data by applying SFBC.

On the other hand, the BS may transmit various types of confirm messagesincluding allocation types of radio resources, which are allocatedaccording to the report type and existence of error in an uplink controlsignal transmitted by the UE, to the UE.

Table 3 shows an example of a confirm message. Here, the report type ofan uplink control signal is assumed to be ‘B’ shown in Table 1. Aconfirm message may be configured with a combination of indication bitsindicating confirm, RI, PMI, and the like.

TABLE 3 Confirm Number of bits message (Indication No error is An erroris type bits) detected detected RI a 1 bits Confirm OL(secondary (0~1:Confirm) (RI, SB, WB) MIMO) <1> (reported RI) <1> b 3 bits ConfirmOL(secondary RI error (0~1: Confirm) (RI, SB, WB) MIMO) OL RI (00~11: RI<1> <1> <4> indication) c 2 bits Used PMI OL(secondary Bitmap (0~1:Confirm) (RI, SB + WB) MIMO) error (0~1: PMI (RI, WB) (reported RI)indication) <2> <1> d 4 bits Used PMI OL(secondary OL RI (0~1: Confirm)(RI, SB + WB) MIMO) RI error (00~11: RI (RI, WB) <1> Bitmap indication)<2> error (0~1: PMI <4> indication) e 7 bits Confirm OL (secondary OL RI(0~1: Confirm) (RI, SB + WB) MIMO) and WB (00~11: RI and <1> <1> PMI WBPMI WB PMI <4> indication) (reported WB (0000~1111: PMI PMI) indication)(overriding) <16>

In confirm message type ‘a’, the RI, SB, and WB transmitted by the UEare applied as they are when no error is detected, and a secondary MIMOtransmit scheme is applied based on the RI transmitted from the UE whenan error is detected.

In confirm message type ‘b’, four cases may be added to inform the UE ofan error in the RI or the RI of a secondary MIMO transmit scheme. Aconfirm message may be three bits in total, comprising one bit forindicating confirm and two bits for indicating the RI.

In confirm message type ‘c’, since the UE should be informed of whetherthe PMI is a frequency selective PMI and a frequency flat PMI or only afrequency flat PMI when there is an error in the bitmap, the confirmmessage may be two bits in total, comprising one bit for indicatingconfirm and one bit for indicating PMI.

In confirm message type ‘d’, when there is an RI error in addition to abitmap error or the BS informs the UE of the RI of the secondary MIMOtransmit scheme, the confirm message may be four bits in total,comprising one bit for indicating confirm, two bits for indicating RIand one bit for indicating PMI. In confirm message types ‘c’ and ‘d’having a bitmap error, the latest used PMI is indicated.

In confirm message type ‘e’, a confirm message may express confirm forusing the RI and PMI transmitted by the UE as they are, indication ofconfirm for indicating application of a secondary MIMO transmit scheme,indication of RI of the secondary MIMO transmit scheme and wholebandPMI, overriding of the wholeband PMI, and the like. A wholeband PMI maybe previously determined depending on the rank of the secondary MIMOtransmit scheme, and the BS may indicate any one of PMIs previouslydetermined. The BS may directly specify a wholeband PMI and inform ofthe wholeband PMI using four bits.

The confirm message described above is merely an example, and the numberof bits of the confirm message and contents to be informed to the UE maybe diversely modified. An indicator for confirm, an indicator for asecondary MIMO transmit scheme, and an indicator for a frequency flatPMI may be respectively configured with bits independent from oneanother, or configured in the form of a bit field indicating respectivestates. In addition, the rank information may be expressed using thebits independent from those of the confirm message or those of theindicator indicating a secondary MIMO transmit scheme or a frequencyflat PMI. Or, the RI may be expressed in an implicit method.

Table 4 shows another example of a confirm message.

TABLE 4 Confirm 1 bit 0: Confirm indicator 1: Secondary MIMO transmitscheme (rank and PMI information bits activation) MIMO Rank 2 bit 00:Rank 1 01: Rank 2 02: Rank 3 03: Rank 4 MIMO 4 bit 0000~1111 PrecodingMatrix Indicator

Since the number of supported ranks is determined depending on transmitand receive antennas, the number of bits of MIMO ranks may be changeddepending on the number of supported ranks. For example, a system thatuses two transmit antennas and two receive antennas (2Tx-2Rx) supportsrank 2 in maximum, and a system that uses four transmit antennas and tworeceive antennas (4Tx-2Rx) supports rank 4 in maximum.

Table 5 shows another example of a confirm message. A codebook accordingto a rank may be indicated.

TABLE 5 Confirm 1 bit 0: Confirm indicator 1: Secondary MIMO transmitscheme (rank and PMI information bits activation) MIMO Rank 6 bit000000~001111: Rank 1 Codebook and 010000~011111: Rank 2 CodebookPrecoding 100000~101111: Rank 3 Codebook Matrix 110000~111111: Rank 4Codebook Indicator

The number of bits of PMI may be changed depending on the number ofsupported codebooks. For example, six codebooks at rank 1 and threecodebooks at rank 2 may be used as codebooks for two transmit antennas.Sixteen codebooks may be used at all the ranks as codebooks used forfour transmit antennas. When a wireless communication system that usesfour transmit antennas supports up to rank 4, two bits fordistinguishing ranks and four bits for distinguishing codebooks at eachrank are allocated to indicate a rank and PMI. In the case of a systemsupporting up to rank 2, a rank and PMI may be indicated using fivebits.

Table 6 shows another example of a confirm message. A confirm indicator,a rank, and a PMI may be expressed together.

TABLE 6 Confirm 7 bit 1000000: Confirm indicator 1000001: Secondary MIMOtransmit scheme and 0000000~0001111: Rank 1 Codebook MIMO Rank0010000~0011111: Rank 2 Codebook and 0100000~0101111: Rank 3 CodebookPrecoding 0110000~0111111: Rank 4 Codebook Matrix Indicator

A method of processing allocated radio resources is determined accordingto the type of an uplink control signal related to multiple antennas,which is received from the UE, and existence of error in the uplinkcontrol signal. All control signals are not transmitted to the UE, but aconfirm message is transmitted for the contents that are applied, andthus overhead caused by transmitting control signals may be reduced.Particularly, although a signal to noise ratio (SNR) may be improvedwhen data is transmitted by applying a frequency selective PMI indownlink, overhead caused by downlink control signals is increased sincea plurality of PMIs should be transmitted. A confirm message istransmitted for the frequency selective PMI, and thus downlink overheadis reduced, and throughput of data transmission may be enhanced.

FIG. 6 is a flowchart illustrating a method of determining whether toapply PMI according to an embodiment of the present invention. It isassumed that a UE transmits PMTs for respective subbands to a BS.

Referring to FIG. 6, In step S210, the BS receives feedback data fromthe UE. The feedback data includes a frequency selective PMI that ismost appropriate to a channel environment of the UE and a bitmap thatspecifies subbands having a high CQI value. A frequency flat PMI on thewholeband may be included in the feedback data. The frequency flat PMImay be transmitted on a control channel, and the frequency selective PMImay be transmitted on a data channel.

In step S220, the BS determines whether there is an error in the bitmapof the feedback data transmitted from the UE to the BS.

In step S230, if there is no error in the bitmap, the BS determinedwhether there is an error in the PMI from the feedback data.

In step S240, if there is no error in the PMI, the BS applies theprimary MIMO transmit scheme to allocate radio resources. If there is noerror in the PMI and the bitmap received from the UE, the PMItransmitted by the UE is applied to allocate radio resources. Here, thePMI may be a PMI on the wholeband or a subband, and the PMI on thewholeband or a subband may be applied depending on conditions of thebitmap and PMI error.

In step S250, when there is an error in the bitmap or the PMI, the BSapplies a secondary MIMO transmit scheme to allocate radio resources.The BS informs the UE through a confirm message that the secondary MIMOtransmit scheme is applied and transmits data to the UE using thesecondary MIMO transmit scheme.

In this manner, a PMI to be applied to allocation of radio resources maybe adaptively selected depending on existence of error in uplink controlsignals. Accordingly, a PMI value is not transmitted on a downlinkcontrol signal, but information on the determined PMI is informedthrough a confirm message, and thus overhead incurred by downlinkcontrol signals may be reduced.

FIGS. 7 to 10 are examples of graphs showing throughputs with respect toerrors in feedback data. FIG. 7 shows a graph when moving speed of theUE is 3 Km/h and the error rate of feedback data is 1%. FIG. 8 shows agraph when moving speed of the UE is 3 Km/h and the error rate offeedback data is 10%. FIG. 9 shows a graph when moving speed of the UEis 15 Km/h and the error rate of feedback data is 1%. FIG. 10 shows agraph when moving speed of the UE is 15 Km/h and the error rate offeedback data is 10%. System throughputs depending on existence of aconfirm message are shown.

Referring to FIGS. 7 to 10, the best result is shown when there is noerror (perfect) in the feedback data. A confirm message is transmittedin alternatives 1 and 2, and a confirm message is not transmitted inalternative 3. When there is an error in the feedback data, SFBC isapplied in alternative 1, and previously transmitted CQIs and PMIs areused or SFBC is applied in alternative 2.

Further better results can be observed when a confirm message istransmitted and a secondary MIMO transmit scheme or previouslytransmitted CQIs and PMIs are used, compared with a case where a confirmmessage is not transmitted.

Every function as described above can be performed by a processor suchas a microprocessor based on software coded to perform such function, aprogram code, etc., a controller, a micro-controller, an ASIC(Application Specific Integrated Circuit), or the like. Planning,developing and implementing such codes may be obvious for the skilledperson in the art based on the description of the present invention.

Although the embodiments of the present invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible; withoutdeparting from the scope of the invention. Accordingly, the embodimentsof the present invention are not limited to the above-describedembodiments but are defined by the claims which follow, along with theirfull scope of equivalents.

1. A method of transmitting data in a wireless communication system, themethod comprising: receiving feedback data on an uplink data channel,the feedback data comprising a preceding matrix indicator (PMI), whereinthe value of the PMI corresponds to an index in a codebook; transmittinga preceding scheme for downlink data on a downlink control channel,wherein the preceding scheme is determined as one of at least two of atransmit diversity irrespective of the received PMI, an acknowledgementindicating preceding according to the received PMI and a new PMIindicating that it is used in preceding downlink data to be transmitted;and transmitting the downlink data on a downlink data channel afterapplying precoding according to the determined preceding scheme.
 2. Themethod of claim 1, wherein the precoding scheme is determined as thetransmit diversity when error in the feedback data is detected by usingcyclic redundancy check (CRC) attached to the feedback data.
 3. Themethod of claim 1, wherein the preceding scheme is determined as the newPMI indicating that it is used in precoding according to the latestreceived PMI.
 4. The method of claim 1, wherein the transmit diversityincludes a space frequency block code (SFBC) and a cyclic delaydiversity (CDD).
 5. The method of claim 1, wherein the feedback datacomprise a plurality of PMIs, each of the plurality of PMIs beingselected from the codebook for each subband.
 6. The method of claim 1,wherein the feedback data comprise a frequency flat PMI on a pluralityof subbands and a frequency selective PMI on selected subbands of theplurality of subbands.
 7. A method of processing data in a wirelesscommunication system, the method comprising: configuring feedback datacomprising at least one PMI, wherein the value of a PMI corresponds toan index in a codebook; reporting the feedback data on an uplink datachannel; receiving a preceding scheme for downlink data on a downlinkcontrol channel, wherein the preceding scheme is determined as atransmit diversity irrespective of the reported PMI or a precedingmatrix which is used to precode the downlink data; and receiving thedownlink data on a downlink data channel.
 8. The method of claim 7,wherein the feedback data comprise a plurality of PMIs on a plurality ofsubbands and one wholeband CQI on the plurality of subbands.
 9. Themethod of claim 7, wherein the feedback data comprise a first PMI on aplurality of subbands, a second PMI on selected subbands of theplurality of subband, one wholeband CQI on the plurality of subbands andone CQI on the selected subbands of the plurality of subbands.